For humans in space, there are two limiting factors namely the microgravity and the galactic cosmic radiation. For sake of better risk assessment for the astronaut, the cellular response to such ionizing radiation qualities, as predominate in Space, needs to be better understood. One of the key elements to be investigated is the cell cycle control as a central element in DNA damage response being a central switch for cellular decision making between life and death. Thus cell death bypass mechanisms in DNA damage response of mammalian cells will be investigated after heavy ion exposure relevant for the Space radiation environment. The RBE-LET dependency will be determined for different biological endpoints. Of special interest are cellular survival, the activation of transcription factors and the identification of genes expressed in DNA damage response after high LET exposure.